Portable electronic devices are becoming smaller and smaller, which accordingly causes the volume and capacity of their batteries to decrease. In order to improve endurance of the batteries, power supply modules of these electronic devices are required to provide a stable output voltage when the battery voltage varies in a wide range. Buck-boost converters thus are widely used in these applications.
FIG. 1 illustrates a traditional buck-boost converter with four transistors. Energy is stored in the inductor L when the transistors S1, S3 are on and the transistors S2, S4 are off. The stored energy is then provided to a load when the transistors S1, S3 are off and the transistors S2, S4 are on. Since the transistors S1-S4 keep switching during operation, power loss of this traditional buck-boost converter is large.
In order to reduce the power loss, different working modes, such as BUCK mode, BOOST mode and BUCK-BOOST mode are adopted. Nevertheless, how to make sure the converter work smoothly during various modes becomes a challenge.